The body averages light exposure from artificial and natural sources to measure day length and modulate melatonin.

“we have a process in our brain and body that averages the amount of light that you're seeing both from artificial sources and from sunlight and measures that off, and it's so exquisitely precise that for a given, say, eight-hour day in the spring, 'cause spring in the Northern Hemisphere or elsewhere, you know, days are getting longer, that means that the amount of melatonin is getting progressively less and less and that signal is conveyed to all the systems of your brain and body.”